To solve the question "Which motion does not require force to maintain it?", we will analyze different types of motion and determine whether a force is required to sustain each type.
### Step-by-Step Solution:
1. **Understanding Force and Motion**:
- Force is defined as the rate of change of momentum. Mathematically, it is expressed as:
\[
F = \frac{dP}{dt}
\]
- Here, \( P \) is momentum, and \( t \) is time.
2. **Analyzing Circular Motion**:
- In uniform circular motion, the object moves in a circle at a constant speed. However, the direction of the velocity changes continuously.
- Since the direction of velocity changes, the momentum also changes, which means:
\[
\frac{dP}{dt} \neq 0
\]
- Therefore, a force is required to maintain circular motion.
3. **Analyzing Elliptical Motion**:
- In elliptical motion, the speed of the object varies at different points along the path.
- Since both the magnitude and direction of velocity change, the momentum is not constant:
\[
\frac{dP}{dt} \neq 0
\]
- Thus, a force is also required to maintain elliptical motion.
4. **Analyzing Straight Line Motion**:
- In uniform straight line motion, an object moves with a constant velocity (both speed and direction).
- If the velocity is constant, the momentum remains constant:
\[
\frac{dP}{dt} = 0
\]
- Hence, no force is required to maintain this type of motion.
5. **Analyzing Parabolic Motion (Projectile Motion)**:
- In projectile motion, the object follows a parabolic path, and its velocity changes due to the influence of gravity.
- Since the velocity changes in both magnitude and direction, the momentum is not constant:
\[
\frac{dP}{dt} \neq 0
\]
- Therefore, a force is required to maintain parabolic motion.
6. **Conclusion**:
- After analyzing all types of motion, we conclude that the only motion that does not require force to maintain it is **uniform straight line motion**.
### Final Answer:
The motion that does not require force to maintain it is **uniform straight line motion**.
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